Early in the second part of the story when Yamada was using Kotori's power, he was able to read Kurosaki's mind despite that he is a witch. However, notice that the kiss is an indirect kiss through Shoba, thus prevent the Mind Reading Power from being overwritten with Time Traveling Power. Very well planned indeed!
Why does the Charm Power quickly takes an effect on Asuka despite that it was established to take sometimes for the effect to manifest? Simple, she faked it. This is supported by the fact that she possesses quite knowledges about the witches, and later, by Nene that those charmed by her power are unlikely to put the charmer in such position that will hurt him/her, something the Asuka obviously isn't.
How was Leona able to stay away from the school for a year without problems despite Sarushima risks being expelled? Considering that the Student Council holds the highest power in the school, it can be justified as Yamazaki has forgotten about Leona's existence. As Rika later said: it doesn't matter if you come to school or not, you'll be forgotten anyway.
In one of the early chapters, we are told that Yamazaki loves girls and is completely indifferent to guys, and as such all student council members - with the exception of Miyamura - are girls. Why Miyamura is made an exception is never actually explained, but the answer can still be discerned many chapters later: Yamazaki loved Leona in the past, but had his memories of her wiped. When he first met Miyamura, his Wistful Amnesia probably kicked in; Miyamura does look like his sister (most noteworthily, they have their unusual hair color and their long canine teeth in common), so Yamazaki probably saw something familiar in him and automatically came to trust him.
During the Cultural Festival, Yamada is able to see Ushio's future despite that the latter in under Odagiri's Charm Power at time. However we are then told that a witch can put people under her power with an Indirect Kiss through object an such and aftermath of the fortunetelling Ushio is having an Indirect Kiss with Nene through the Yakisoba Bread Sandwich, hinting that this probably happened before and is the reason why the power is on and off on him.
It is not known at first when did Kotori became close enough with Ushio to lower her defense enough for him to steal her power. However, it is later revealed that she did associate with him one year ago during the previous ceremony (that resulted in mass memory wipe) and Yamada's latest kiss was the reason that her memory was restored! Bingo!
In the epilogue, Sarushima greets the wedding guests with the Italian "ciao", which may seem a bit odd given that she's known for her Gratuitous English and not her Gratuitous Italian. Then you realize that she's become a fashion designer, and that Milan is one of the world's big fashion capitals, making it very likely that she's visited that city several times and picked up some of the language.
